This invention relates to compositions for use as metal hydride electrodes for batteries, and, in particular, to new non-stoichiometric AB.sub.5 class alloys which can be used in such electrodes.
Secondary (rechargeable) batteries are used as a power source for the numerous portable electronic appliances recently developed. There is, therefore, a need for a secondary power source for these electronic appliances which has a high storage capacity and long cycle life. Although a nickel-cadmium battery has been developed for such a purpose, the capacity of this battery has not been significantly increased in recent years. Further cadmium is a highly toxic material and has a very negative environmental impact.
Also, there is an ongoing investigation into the manufacture of batteries that can be utilized in electric motor vehicles or hybrid motor vehicles which can operate on electric or combustion power. In order for electric powered vehicles to be manufactured at a cost that is viable in the marketplace, a less expensive battery must be developed which has a relatively high capacity and cycle life. A major obstacle to manufacturing such a battery has been the need to include cobalt as one of the components in the anode of such batteries. The high cost of cobalt contributes to the prohibitive expense of such motor vehicle batteries. Thus, there is a need for a battery which includes an anode having good electrochemical properties, but does not require cobalt as a component.
The nickel/metal hydride cell is a rechargeable electrochemical storage cell generically known in the art as the "Ni/MHx cell". The Ni/MHx cell has a positive electrode (cathode) which is composed of nickel oxyhydroxide (NiOOH) and an anode composed of a hydrided metal alloy. The cell also contains an ion-conducting electrolyte and a separator which prevents direct contact between positive and negative electrodes.
The half cell reactions taking place in a Ni/MHx cell may be written as follows: EQU MH.sub.x +xOH{character pullout}M+xH.sub.2 O+xe EQU Ni(OOH)+H.sub.2 O+e{character pullout}Ni(OH).sub.2 +OH
It is in effect a rocking chair type electrochemical cell in which hydrogen is transferred from one electrode to the other. It is convenient that the voltage is essentially the same as the conventional Ni/Cd cells.
Ni/MHx cells have similar operating characteristics to nickel/cadmium (Ni/Cd) cells, but the Ni/MHx cell uses a metal hydride anode in place of cadmium. Metal hydrides are attractive as replacements for the cadmium electrode in the nickel-cadmium batteries from both an environmental and performance viewpoint. More specifically, the advantages of Ni/MHx cells include higher specific energy, higher energy density, improved environmental compatibility, and the potential for a longer cycle life.
However, the charge/discharge cycle lifetime of the Ni/MHx batteries has been limited by degradation of the metal alloy in the electrode due to corrosion. The negative electrode containing the hydrogen absorbing alloy is immersed into an alkaline solution when it is assembled into the secondary cell. The anode is also exposed to oxygen evolving from a positive electrode when the battery is excessively charged. Thus, the hydrogen absorbing alloy may corrode causing its electrode characteristics to deteriorate.
At the anode of the nickel/metal hydride cell, a reversible electrode oxidation reaction occurs with OH ions at the surface of a metal alloy. When the battery is charged, a corresponding reduction reaction occurs at the surface of the metal electrode in which hydrogen is absorbed into the alloy producing a solid metal hydride and hydroxide ion. When the electrode corrodes, less hydrogen is absorbed and the electron conductivity is reduced due to the presence of non-hydrogen absorbing insulating corrosion products. The amount of hydrogen absorbed into the AB.sub.5 electrode during the charging process is termed storage capacity. Storage capacity is equal to the ability of the AB.sub.5 electrode to discharge electrons, also called discharge capacity. The discharge of electrons is measured in milliampere hours per gram of alloy material, mAh/g.
The metal unit cell expands when absorbing the hydrogen and shrinks when releasing the hydrogen. The increase in volume during the hydriding reaction is termed the atomic volume of hydrogen, V.sub.H. This process has been directly correlated to electrode corrosion. See, Willems J J G and Buschow K H J, J. Less-Common Metals, 129:13 (1987). The anode is therefore subjected to volumetrically induced strains during charging and discharging cycles. For example, upon the formation of LaNi.sub.5 H.sub.6 the alloy expands in volume by approximately 24 percent. There is also a corresponding contraction when hydrogen is removed. This imposes great mechanical stress on the alloy which, consequently, pulverizes into fine particles. Furthermore, large volume changes in each charge and discharge cycle increases the flushing action of the electrolyte through the pores and micro-cracks of the electrode, thereby increasing the corrosion rate.
Thus, the alloy of the electrode must resist corrosion due to oxidation, while maintaining a high level of storage capacity.
Currently, there are two types of alloys which are of interest as metal hydride electrodes, the AB.sub.5 and the AB.sub.2 classes of intermetallic compounds. The AB.sub.5 class of compounds have a hexagonal CaCu.sub.5 structure where the A component comprises one or more rare earth metals and B consists of Ni, or another transition metal or a transition metal combined with other metals. The AB.sub.2 alloys are Laves phases with as many as nine metal components. Alloy formulation is primarily an empirical process where the composition is adjusted to provide one or more hydride forming phases in the particle bulk, but has a surface that is presumed to be corrosion resistant because of the formation of semi-passivating oxide layers. Unlike the AB.sub.5 alloys, there are few systematic guidelines which can be used to predict alloy properties. Therefore, the potential use of the AB.sub.2 alloy in electrodes is far from realized.
The paradigm compound of the AB.sub.5 class of compounds is LaNi.sub.5. However, LaNi.sub.5 is not a suitable electrode because the hydride is too unstable and the alloy corrodes rapidly in the chemically aggressive battery environment. Alloy modifications to LaNi.sub.5 have been made by substituting other elements for lanthanum, nickel, or both, and by changing the overall stoichiometry of the alloy. However, improvements in the charge/discharge cycle lifetime by the introduction of additional metal components into the alloy are usually accompanied by a reduction in the hydrogen capacity of the metal alloy.
The composition of commercial AB.sub.5 electrodes revolve around a formula first suggested by Ikowa M, Kawano H, Matsumoto I, and Yanagihara N, Eur. Patent Appl. #0271043 (1987), MmB.sub.5, in which mischmetal, a low cost combination of rare earth elements (predominantly La, Ce, Pr and Nd), is used as a substitute for La. The B.sub.5 component remains primarily Ni but is substituted in part with Co, Mn, Al. The partial substitution of Ni increases the thermodynamic stability of the hydride phase as well as its corrosion resistance. The AB.sub.5 composition in commercial batteries is variable, but most have a composition similar to MmNi.sub.3.55 Co.sub..75 Mn.sub..4 Al.sub..3. Such electrodes have demonstrated a good storage capacity and cycle life. Ikowa et al. (1987).
The efficacy of this remedy has been attributed in part to the reduction of the molar volume of hydrogen in the hydride phase thereby reducing alloy expansion and contraction during the charge-discharge cycle. This, in turn, leads to a reduction of the flushing action of the electrolyte through the small pores and fissures of the alloy produced in the initial activation process. Willems and Buschow (1987). Consequently, corrosion of the electrode is reduced.
Previous work has shown that the presence of cobalt in AB.sub.5 alloy electrodes significantly reduces corrosion both by reducing V.sub.H (See, Adzic G, Johnson J R, Mukerjee S, McBreen J, Reilly J J, J. Alloys and Compounds 579:253-254 (1997)) and by the formation of a corrosion inhibiting surface layer (See, Mukerjee S, McBreen J, Adzic G D, Johnson J R, Reilly J J, Extended Abstracts, National Meeting of the Electrochemical Society, San Antonio, Tex., USA, Abstract #48, 96-2, 1996). In addition, there is evidence that cobalt in the alloy lowers the hardness or stiffness of the lattice structure resulting in reduced pulverization when cycled. See Chartouni D, Meli F, Zuttel A, Gross K, Schlapbach L, J. Alloys and Compounds, 241:160 (1996).
Currently, cobalt is present in practically all AB.sub.5 electrodes used in Ni/MHx batteries. It tends to increase hydride thermodynamic stability and inhibit corrosion. However, it is also expensive and therefore substantially increases battery costs.
Thus, there remains a need for an AB.sub.5 structural type alloy which can be used as as a battery electrode that demonstrates a high storage capacity while also having a good cycle life. It is preferable that such an alloy be created without having to utilize cobalt, due to its high cost.
Toward this end the present invention provides a new non-stoichiometric AB.sub.5 structural type alloy which can be utilized as an electrode to improve the storage capacity and cycle life of an electrochemical cell or battery which incorporates said electrode.